It has been heretofore known to add viscosity index improver comprising polyalkyl(meth)acrylate to lubricating oil. For example, U.S. Pat. No. 2,628,225 discloses to add a polyalkylmethacrylate to a lubricating oil. JPN Patent Laid-open No.12982/1972 discloses a viscosity index improver comprising copolymer having alkyl acrylate and alkyl methacrylate structural units. Polyalkyl(meth)acrylates have been used widely, since they have good viscosity index improving ability, low-temperature viscosity characteristics and accompanied by pour point depressing ability. Polyalkyl(meth)acrylates employed heretofore contain at least 50% by weight of alkyl (meth)acrylate structural units having at least 12 carbon atoms in the alkyl group in order for them to possess pour point depressing ability. Alkyl methacrylates are mainly used as the structural units for providing much of the pour point depressing ability.
As to lubricating oils and hydraulic fluids, it is desired that their low-temperature viscosity, as well as their cost, be low, so as to meet economic needs. As a means for accomplishing this, high viscosity index oils containing iso-paraffins, synthetic lubricating oils, MLDW oils and the like are coming into wide use, and these have been used alone or in combination with usual solvent-refined mineral oils. Particularly, high viscosity index oils are becoming widely used in view of performance and cost. To use such high viscosity index oils, it has been proposed to reduce low-temperature viscosity by combining them with a pour point depressant. This is so disclosed, for instance, in JPN Patent Lay-open No. 70305/1979. There is thus a demand for a viscosity index improver suitable for these high viscosity index oils. Also, for reducing fuel cost, a molybdenum-containing friction modifier or anti-wear agent (FM agent) has been used.
Particularly, for lubricating oils for passenger cars it is necessary to improve their low-temperature viscosity characteristics and oxidation resistance, yet be of low fuel cost, in order to comply with recently established CAFE regulations and lubricating oil standards. Known polyalkyl (meth)acrylate viscosity index improvers, are insufficient to satisfy these desired objectives of improving low-temperature viscosity characteristics and oxidation resistance and yet be of low fuel cost. Especially, in engine oils, gear oils and automatic transmission oils, attaining these objectives is desired and essential.
In addition, since known polyalkyl (meth)acrylate viscosity index improvers are of insufficient oxidation resistance and heat resistance and have poor solubility and low-temperature viscosity characteristics in high viscosity index oils and synthetic lubricating oils, good oxidation resistance and heat resistance of these base oils cannot be obtained thereby. Particularly in high viscosity index oils, known viscosity index improvers are insufficiently effective with respect to low-temperature viscosity characteristics.
Furthermore, when known viscosity index improvers and FM agents are added to lubricating oils they are subject to oxidative degradation, their friction coefficient is sharply increased, and friction wear reducing effects are not fully attained.
Applicants have discovered that a viscosity index improver comprising a polymer containing structural units of alkyl (meth)acrylate monomers having alkyl group containing a specific numbers of carbon atoms provides improved viscosity index improving ability, low-temperature viscosity characteristics and reduced fuel cost together with improved oxidation resistance, and provides for full effects of FM agents.